1. Field of the Invention
The present invention relates to novel methods for the anaerobic digestion of various solid waste feedstocks, including but not limited to, municipal, industrial, and agricultural wastes, biomass feedstocks including marine, woods, and grasses, and fossil feeds such as coal and peat.
2. Discussion of the Prior Art
Sanitary landfills formed by filling a land area with successive layers of solid waste, principally household waste, and layers of earth or soil are well known. The uncontrolled landfill depends upon natural biological action, precipitation and climate to effect decomposition. In areas where oxygen is present, the decomposition will be aerobic and in areas where little oxygen is present, such as at the deeper depths, decomposition will be slower and anaerobic, producing methane-containing gas. Initially, there is no methane production from the landfill and the eventual total gas production from a landfill, as compared with the total potential production, may vary widely depending upon whether the landfill has been properly designed. Total gas production can range from being extremely limited in terms of total potential gas production, to being essentially complete (i.e. close to the theoretical potential production). The formed methane is an explosion or fire hazard and may migrate to buildings or structures several hundred feet from the landfill if not removed from the landfill. Further, the natural precipitation draining out of the landfill may carry toxic contaminated water to contaminate underground water supplies, surface streams and wells. Due to the very slow stabilization, the uncontrolled landfill is not usable for other purposes for long periods of time and, thus, particularly near metropolitan areas, represents a large waste of land resources.
One approach to rendering waste disposal landfills safer is suggested by U.S. Pat. No. 3,586,624, which teaches a liquid-impervious containment of the lower portion of the landfill with continuous flow of water through the landfill to accelerate the decomposition, decrease the fire hazard and flush contaminants from the landfill in a controlled manner. The water drained from the landfill may be treated for removal of contaminants and recycled to the landfill.
In the past, methane gas has been frequently vented and flared from landfills as a safety precaution. However, in recent years and especially in view of energy conservation, the recovery and utilization of methane from sanitary landfills and the desirability of early utilization of the landfill area for other purposes has been recognized [James et al., "Methane Production, Recovery and Utilization from Landfills," Symposium Papers on Energy from Biomass and Wastes, Washington, D.C., Aug. 14-18, 1978, pages 317-324; and Stearns et al., "Recovery and Utilization of Methane Gas From a Sanitary Landfill--City of Industry, California," Symposium Papers on Energy from Biomass and Wastes, Washington, D.C., Aug. 14-18, 1978, pages 32--343]. Presently, methane is most frequently recovered from landfills by pipes extending into the landfill and transporting the methane-containing gas formed within the landfill to a collecting area for further treatment.
In the United States, millions of tons (dry) of organic wastes are generated annually in the form of municipal solid waste, agricultural residue, manure, logging and wood manufacturing residues, municipal sludge solids, industrial organic waste and miscellaneous organic waste representing production potential of trillions SCF/year of substitute natural gas (SNG). The most readily available solid waste for energy recovery is generated at about 260 million tons per year in the United States.
The biodegradable "natural" fraction of typical U.S. municipal solid waste makes up about 67% (dry or wet weight basis) of the total weight of this waste stream and is composed of approximately 41% paper, 18% yard wastes and 8% food wastes (Franklin Associates, 1988). Factors which prevent direct land disposal of this fraction are: 1) contamination with non-natural toxic or refractory components, 2) unsightly appearance in its unprocessed form, 3) production of organic acids and odors during decomposition of the rapidly biodegradable fraction, and 4) attraction of pests and spread of disease. When separated from the undesired components (hazardous wastes, plastics, metals, glass, fabrics, etc), and shredded and biologically treated to remove rapidly biodegradable components, the resulting compost product is not only suitable for land disposal, but significantly improves the water-retaining capacity of the receiving soil if the soil is light (sandy). Compost also improves aeration in heavy (clay) soils. This method of treatment provides the opportunity for integration of a major fraction of the waste stream into the natural stream for cycling of the elements in the biosphere.
The biological process applied to degradation of the rapidly biodegradable fraction of solid wastes is commonly termed "composting." Composting is normally considered to be an aerobic process accomplished either by mixing or forced aeration methods. Technically, however, the term "composting" addresses the fact that a biological stabilization process has occurred resulting in a product that has properties making it valuable as a soil amendment. The term is furthermore used most widely for treatment of feedstocks with a high solids content. It is more correct to use the term "composting" in association with anaerobic digestion when applied to conversion of high solids feeds to compost and other products [DeBaere et al., Resources and Conservation, Vol. 14, pages 295-308 (1987)].
Diversion of the biodegradable fraction of municipal solid waste and its treatment by composting is receiving increased attention. If potentially toxic contaminants are removed by source separation or other separation techniques, the residues of composting do not pose an environmental threat and may be disposed of, or even marketed as a compost soil conditioner. Since the biodegradable fraction composes about 67% of a typical MSW stream, any effective treatment of this fraction in a manner that diverts it from landfills will have a major impact on solving the problem of disposing of MSW.
The term "composting" is associated with either in-vessel or out-of-vessel processes in which oxygen is provided by mixing and/or by application of air to enhance decomposition. Anaerobic composting (more commonly referred to as "anaerobic digestion") accomplishes similar extents and rates of decomposition without the need for aeration or mixing (some designs). It not only has reduced energy requirements, but also produces the valuable energy product methane.
As noted previously, anaerobic decomposition in the form of a methane fermentation occurs naturally in landfills; however, decomposition is slow (requiring years) and often incomplete. This slow rate may be attributed to lack of organisms, moisture and nutrients necessary for rapid fermentation. Various digester designs have been developed and tested at different scales for anaerobic composting of MSW. The more attractive options process the feedstock in its high-solids form to minimize reactor size and the possible energy penalties associated with heating water. These digester designs have included mixed, plug-flow, batch and multi-stage with leachate recycle.
Although application of anaerobic composting to high solids feeds is less developed commercially than aerobic composting, it effects equivalent conversion at similar retention times and produces a compost of equivalent quality. Capital costs of anaerobic composting are expected to be similar to those of in-vessel aerobic composting, since front and end processing operations are similar and vessel designs are not substantially different. The major advantages of anaerobic composting over aerobic composting is the lack of need for aeration or mixing and the production of a valuable fuel gas in addition to compost. For example, 100 metric tons of MSW will yield about $1,300 worth of methane (assuming $3 GJ.sup.-1) and about $300 worth of compost (assuming $10 ton.sup.-1 compost). This indicates that the methane credit is significantly greater than that of compost and results in an economic advantage for anaerobic over aerobic composting of about $13 per ton MSW processed. Other advantages of anaerobic composting are maintenance of nitrogen in the reduced state and lack of odors associated with partially aerobic and partially anaerobic conditions associated with outdoor aerobic composting.
Anaerobic bioconversion of the organic fraction of municipal solid waste may be considered an attractive option for inclusion in an integrated solid waste management program. The process produces a medium BTU gas without creating the air pollution problems associated with incineration. In addition, it minimizes leachate management problems and reduces the amount of solids for ultimate disposal.
Conventional mixed one-step digesters require feed material with a total solids (TS) content below 15%. However, a number of farm and municipal solid wastes have solids contents exceeding 30%. Utilizing conventional digesters could, therefore, result in a substantial volume increase compared to high-solids reactors. The main problems with low solids reactors applied to high-solids feedstocks are in the materials handling of the reactor slurry in an unmixed reactor, or prohibitive mixing energy requirements in a mixed reactor. Because of these problems, many scientists have focused their research on the development of digester designs which would effectively process high solids feedstocks.
Batch reactors may be desirable for high solids feeds because of difficulties of moving and mixing their materials. However, batch anaerobic digesters can be expected to have a high volatile fatty acid build-up in the start-up phase which leads to a drop in pH and an inhibition of methanogens. Research by a number of scientists has concentrated on overcoming these disadvantages.
Keenan [J. Environmental Scientific Health, All, Vols. 8 and 9, pages 525-548 (1976)], Rijkens ["A Novel Two-Step Process for the Anaerobic Digestion of Solid Waste" in Energy from Biomass and Wastes V, Symposium Papers, pages 463-475, Lake Buena Vista, Fla. (1981)], Barry-Concannon et al., [Energy From Biomass, Vol. 3 (1983)], Chynoweth et al., [Proc. 20th Intersoc. Energy Conversion Eng. Conf., pages 1.573-1.579, Miami Beach, Fla. (1985)], Smith et al., ["Biological Production of Methane from Biogas" in Methane From Biomass: A Systems Approach, pages 291-331, ed. W. H. Smith et al., Elsevier App. Sci., London (1988)], and Jewell et al., ["Engineering Design Considerations and Methane Fermentation of Energy Crop," from Annual Reports submitted to Gas Res. Inst., Chicago, Ill., Contract No. 5083-226-0848 (1983-1987)] have developed multi-stage anaerobic digesters in which the hydrolysis/acidification phase and the subsequent methanogenic phase are optimized in separate reactors and a better overall process stability is achieved. Smith and Jewell used a bench-scale leach bed/packed bed system. The leach bed batch reactor was filled with plant material such as Napiergrass, water hyacinth shoots, straw with cattle manure, or wood and, through leach bed management, the acids produced were transported to a packed bed digester and converted into biogas by a methane phase. Barry-Concannon preferred an up-flow anaerobic filter instead of a packed bed digester for the second stage.
Rijkens ["Two-Phase Process for the Anaerobic Digestion of Solid Waste; First Results of a Pilot-Scale Experiment" in Energy From Biomass, 2nd E. C. Conference Proceedings, Berlin (1982)] and Jewell scaled up their previous bench-scale designs to pilot scale. Rijkens used a 75 m.sup.3 leach bed to acidify waste tomato plants and converted the acids to biogas in an up-flow anaerobic sludge blanket (UASB) reactor. Jewell scaled up the reactor to 2.4 m.sup.3. They monitored five phases of digestion: 1) ensiling, 2) leaching, 3) inoculation, 4) independent operation, and 5) long-term batch operation. The leachate was converted into methane in a secondary reactor which contained partially digested sorghum removed previously from the first reactor.
Hall et al. ["Operation of Linked Percolating Packed Bed Anaerobic Digesters" in Fifth International Symposium in Anaerobic Digestion, Bologna, Italy (1988)] went one step further and used secondary and even tertiary digester reactors to produce biogas. These reactors were loaded earlier and had gone through the same phases as the newly loaded primary reactor. Leachate was percolated through a mixture of wheat straw and dairy manure in the primary digester. Subsequently, percolate was pumped into the secondary reactor and then into the tertiary, where enough methanogens had accumulated to convert the leached acids into biogas. To complete the cycle, leachate was returned back into the newest digester in order to leach out more acids and inoculate the feed with methanogens.
Having successfully demonstrated that leachate management achieves stable conversion in batch digesters filled with high solids plant biomass, Ghosh ["Solid-Phase methane Fermentation of Solid Wastes" in Eleventh American Soc. of Mech. Eng. Natl. Waste Processing Conference, Orlando, Fla. (1984)] used successfully a leach bed/packed bed concept in a bench-scale system for the breakdown of RDF (refuse-derived fuel) into biogas and stabilized residue.
Particular problems encountered in attempts to design full-scale operating units employing anaerobic composting, particularly units to be designated to handle MSW, are the sheer size requirements for providing the necessary capacity to process the MSW generated by a given municipality, and the capacity limits and operating economies of materials handling equipment required to transport and deposit the MSW in the holding cells used for composting, and to remove and transport away the compost after processing.
It is an object of the present invention to provide a novel improved apparatus and method for the anaerobic bioconversion of MSW to compost and methane.